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  • The Utility of Lactate Dehydrogenase Isoenzyme Pattern

The Utility of Lactate Dehydrogenase Isoenzyme PatternINTRODUCTION

The lactate dehydrogenase (LDH) molecule is a tetramer composed of four polypeptide chains. There are five component isoenzymes as a result of the five different combinations that are produced by two polypeptide chains encoded by separate genes (M and H). LDH-1 is composed of four H subunits, and LDH-5 of four M subunits. The prevailing type of LDH varies according to tissue type. In the heart, the H gene is more active than the M gene, the latter being strongly expressed in the skeletal muscle. As the number of the M over H chains increases, the LDH isoenzyme becomes more efficient in catalyzing the conversion of pyruvate to lactate (LDH-5), while an increase in H over M chains (LDH-1) favors the conversion of pyruvate to acetyl-coenzyme A that enters into the citric acid cycle. Serum LDH isoenzymes may be useful in differential diagnosis of ascites etiology since tissue damage releases isoenzymes contained therein, leading to a change in their pattern.

Ascites can present a challenging diagnostic problem. The differential diagnosis is diverse, but most common causes include cirrhosis, spontaneous bacterial peritonitis, tuberculosis, congestive heart failure and malignancy. A complete separation between malignant ascites and nonmalignant ascites has not been always possible. Exfoliative cytology for malignant cells despite its high specificity is unreliable as positive results are obtained only in 40-60% of cases. Therefore, LDH isoenzyme analysis may contribute to cytologic evaluation of serosal fluid and may suggest a malignant etiology even in cytologically negative neoplastic pleural effusions. Measurements of ascitic fluid LDH levels are useful as one of the parameters in the separation of exudative from transudative effusions. In addition to their higher enzyme concentrations, many of the tissues show different isoenzyme composition. In cardiac muscle, kidney and erythrocytes, the electrophoreti-cally faster-moving isoenzymes LDH-1 and LDH-2 predominate, whereas in liver and skeletal muscle the more cathodal LDH-4 and LDH-5 isoenzymes predominate (although skeletal muscle may also contain midzone and anodic LDH isoenzymes). Isoenzymes of intermediate mobility are found in many tissues, such as endocrine glands, spleen, lung, lymph nodes, platelets and nongravid uterine muscle. The value of serosal LDH isoenzyme pattern is controversial.
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Ascitic cytology is highly specific but has a diagnostic sensitivity of only 40-60%. No laboratory test completely distinguishes malignant ascites from ascites associated with cirrhosis or other benign etiologies. The objective of the present study was to determine the ascitic fluid LDH level and LDH isoenzyme activities in patients with malignant and nonmalignant ascites (spontaneous bacterial peritonitis, sterile cirrhotic ascites, tuberculous ascites, congestive heart failure-related ascites) to see whether ascitic fluid biochemical examinations might help in the analysis of ascites.
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